Method and apparatus for reducing set-down of a tension leg platform

ABSTRACT

An Auxiliary Mooring System (AMS) supplements a Tension Leg Platform (TLP) tendon-based mooring system, to provide lateral resistance to offset movement of the TLP in extreme survival events with minimum impact to the TLP or permanent tendon system design. Limiting platform offset also limits set-down of the platform. The mooring system of the invention provides a method and apparatus for supplementary mooring of a Tension Leg Platform. The apparatus includes a plurality of catenary mooring lines attached to the platform&#39;s lower columns or pontoons and secured to the seabed by means of piles or other established anchoring means. The method of the AMS includes installing compensating buoyancy devices on the platform.

CROSS-REFERENCE TO RELATED APPLICATIONS

none

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to offshore platforms. More particularly, it relates to a supplemental anchoring system for deepwater tension leg platforms.

2. Description of the Related Art

A tension-leg platform (TLP) is a vertically-moored, floating structure normally used for the offshore production of oil and/or gas, and is particularly suited for water depths greater than about 1000 feet. A TLP typically comprises a buoyant hull that is at least partially submerged and one or more decks supported above the surface of the water by the hull.

The TLP is connected to a fixed foundation (or piles) by pre-tensioned tendons. The tendons are normally parallel, near vertical elements, acting in tension, which usually restrain the vertical plane motions of the TLP in heave, roll and pitch. The platform is usually compliant in surge, sway and yaw. FIG. 1 shows an example of a tension leg platform.

The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners or at the ends of pontoons extending from a column-shaped hull. A feature of the design of the tendons is that they have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the undersea wells by rigid risers), instead of on the seafloor. This makes for less expensive well completions and workovers and allows better control over the production from the oil or gas reservoir. The platform is also an excellent facility for subsea wellhead applications and Steel Catenary Risers due to the reduced platform motions.

Although a TLP permits almost no vertical motion, it can move sideways under the influence of wind, waves and ocean currents. As is illustrated in FIGS. 1 and 2, lateral movement of TLP 10 in response to Force F to displaced TLP 10 a results in set-down—a decrease in the distance between the surface of the sea W and the lowest deck of superstructure 20—owing to the geometry of the tendons 16, the tendon anchors in seafloor S and the tendon attachment points on the pontoons 14 of the TLP. The hull of a TLP is commonly designed such that its buoyant force increases with lateral displacement (and set-down) of the platform. This effect generates a restoring force which tends to move the platform back into a position directly above the tendon anchor points with the tendons oriented vertically. However, the force of the wind, waves and/or current may be such that the equilibrium point of the platform is displaced from the vertical and hence there is some degree of set-down of the platform.

TLP's, like all other offshore platforms, must be designed to withstand certain extreme environmental conditions that may be encountered during a storm or hurricane. Commonly, these conditions are defined in terms of the strongest storm likely to be experienced by the platform in a certain time period—e.g., “a 100-year storm” or “a 1000-year storm.” Perhaps the most important environmental criterion that a TLP must be able to withstand is wave height. The height of the lowest deck must be sufficient to clear the highest wave likely to be encountered. As explained above, lateral displacement of a TLP results in set-down of the platform which reduces the height of the deck(s) above the water. Limiting the lateral displacement of a TLP therefore reduces the deck height necessary to meet the worst-case environmental scenario. This both enhances the safety of the platform and reduces its size and cost.

The TLP encounters increasing design challenges if extended into very deep water and under harsh environmental conditions. Firstly, tendon weight must increase as water depth increases in order to maintain adequate tendon stiffness. This is typically achieved by increasing tendon cross section, which means increased tendon weight, requiring more buoyancy and hence a larger hull, which in turn boosts tendon stiffness requirements. Secondly, the requirements regarding air gap increase, owing to the set down effect. Air gap is the distance between the surface of the water and the underside of the lowest deck of the TLP. Both of the aforementioned characteristics are a negative effect both technically and commercially. Furthermore, the number of suppliers of large diameter thick-walled high-strength tendons is extremely limited and can easily result in a single worldwide source, which may result in prohibitive costs.

In recent years there has been an increasing number of severe weather events, and the resulting substantial platform damage has called into question the validity of the survival event design data that has been employed by the industry. A TLP is more sensitive to changes in survival weather criteria than traditional catenary-moored vessels. This can be compensated for in the design phase of a TLP through adequate weight management and tendon detail design optimization, but at a significant cost. One impact of an increased survival event is to cause significant increases in TLP offset (surge and sway) and set-down, and results in substantially higher maximum tendon tensions and angles, and the potential for a slack tendon condition.

An auxiliary mooring system is a cost-effective method of supplementing either existing, operational TLP'S, those in mid-fabrication cycle, or new build designs, in order to meet new, enhanced-survival events.

BRIEF SUMMARY OF THE INVENTION

An auxiliary mooring system is used to constrain the offset (surge and sway) of a tension leg platform. The auxiliary mooring system additionally provides a redundant restraint system for the platform in the event of tendon failure. In one embodiment, the auxiliary mooring system comprises a plurality of catenary lines secured by anchors on the seafloor. Combined fairleads/chain stoppers mounted on submerged pontoons of the TLP may serve as the termination point of the mooring lines on the TLP. Additional buoyancy devices may be attached to the TLP to compensate for the weight of the mooring lines. Subsurface buoys attached at selected locations on the mooring lines may be used to further direct and modify the forces exerted on the TLP by the mooring lines. The auxiliary mooring system of the invention may also be used to retrofit existing tension leg platforms.

The Auxiliary Mooring System (AMS) of the present invention has been developed so that the hurricane survival event does not dictate the tendon design and instead the tendon system works in conjunction with a catenary mooring system to provide a composite and cost-effective means of platform vertical and lateral restraint. The AMS also provides additional restraint in deep water under the enhanced-survival event.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 depicts the set-down phenomenon in a tension leg platform of the prior art.

FIG. 2 is an enlarged view of the upper sections of the TLP depicted in FIG. 1.

FIG. 3 illustrates a TLP having an auxiliary mooring system according to a first embodiment the present invention.

FIG. 4 illustrates a TLP having an auxiliary mooring system according to a second embodiment of the invention which comprises subsurface buoys on the mooring lines.

FIG. 5 shows one method for installing an auxiliary mooring system on a TLP using a surface vessel.

FIG. 6 depicts an alternative method for installing an auxiliary mooring system of the invention.

FIG. 7 shows several views and embodiments of chain stopper/fairlead devices for terminating a mooring line on the pontoon of a TLP.

FIG. 8 shows an embodiment wherein the auxiliary mooring system includes added buoyancy devices on the TLP.

FIG. 9 a is a partially-sectioned side view of a TLP equipped with an auxiliary mooring system according to an embodiment of the invention which comprises a chain chute through each pontoon of the TLP.

FIG. 9 b is a side view of the pontoon end of the TLP shown in FIG. 9 a.

FIG. 9 c is a plan view thereof.

DETAILED DESCRIPTION OF THE INVENTION

A Tension Leg Platform Auxiliary Mooring System according to the present invention may comprise a floating platform secured by a plurality of vertical tendons comprising steel pipe with an effective diameter-to-wall thickness ratio that provides a nearly neutrally buoyant condition. The system may also be used in conjunction with tendons having neutral, positive or even negative buoyancy. The lower ends of the tendons may be anchored to the sea bed and extend to near the sea surface while being held in tension by the buoyant floating platform such that the platform stays in a near vertical position. The mooring system additionally comprises a plurality of platform auxiliary moorings arranged in a catenary pattern extending a distance from the platform to seabed anchors.

Referring now to FIG. 3, a TLP 10 comprising hull 12 equipped with a plurality of wings or pontoons 14 is secured by tendons 16 secured to piles 24 in seafloor S. Wings of pontoons 14 may be buoyant structures. Superstructure 20, which may comprise a plurality of decks, is attached to hull 12 by means of jacket structure 22. Risers 18 (shown in FIG. 1) connect equipment on the deck(s) to well heads, pipelines and related apparatus on seafloor S.

For the single-column TLP design shown in FIG. 3, a single catenary mooring line (26) may be terminated at the end of each pontoon (14), providing an auxiliary, multi-leg, symmetrical or non-symmetrical, catenary mooring system. The seabed termination of the mooring lines may be the same system as that used by the tendons—i.e., driven piles. Alternatively, the anchoring means 28 for the mooring lines may comprise suction anchors, vertically loaded anchors (VLAs) or similar devices. The connection of mooring line 26 to TLP 10 at pontoon end 32 may be significantly simplified compared to systems of the prior art because it requires no operational adjustment, tension monitoring equipment or chain lockers.

As illustrated in FIG. 4, mooring line 26 may be equipped with subsurface buoy 30 to at least partially support line 26 in multiple catenary segments and modify and direct the forces applied to TLP 10 by mooring line 26. Mooring line 26 may be equipped with multiple subsurface buoys. The attachment means of subsurface buoy 30 to mooring line 26 may comprise a triplate or other means well-known in the art.

Mooring line 26 may comprise chain sections, wire rope sections, and/or synthetic fiber sections. In some embodiments, lines 26 may be formed of a single material. In one preferred embodiment, at least the lower end of line 26 comprises heavy chain which rests on seafloor S when TLP is in a position such that tendons 16 are vertically oriented. Lateral displacement of TLP 10 from this position tightens the upwind or upstream mooring line(s) 26 which tends to raise the chain section at the seafloor end. The force of gravity acting on the raised chain section provides a restoring force to TLP 10, urging it back towards its original position wherein the tendons 16 are vertical. The water weight of line 26 may be compensated for by means of additional buoyancy in hull 12. Alternatively, additional buoyancy devices may be added to hull 12 to at least partially compensate for the downward component of force exerted on TLP 10 by lines 26. One such embodiment comprising additional buoyancy devices is shown in FIG. 8.

The embodiment illustrated in FIG. 8 comprises pontoon extensions 52 on the distal ends of pontoons 14. Pontoon extensions 52 may be located outboard of tendon porches 38. Alternatively, pontoon extension 52 may be attached to the side(s), bottom or top of pontoon 14. In certain embodiments, it may be advantageous to attach a plurality of pontoon extensions 52 to a plurality of locations on pontoon 14. Fairlead 40 or a combination chain stopper/fairlead may be attached to the outboard end of pontoon extension 52 to provide a termination of mooring line 26.

The embodiment illustrated in FIG. 8 is particularly well-suited for adding an auxiliary mooring system to an existing TLP design inasmuch as some or all of any additional hull buoyancy necessitated by the addition of the AMS may be provided by the pontoon extensions 52. Moreover, the additional buoyancy is provided close to the attachment points of the mooring lines 26. This feature also helps to avoid having to change any of the existing design parameters of the TLP.

Yet another embodiment of a pontoon extension 52 is shown in FIGS. 9 a, 9 b and 9 c. This embodiment employs a flared chain chute 54 through the interior of each pontoon extension. Chain chutes 54 may obviate the need for fairleads 40. Chain stopper 48 may be located at the upper end of chain chute 54 to provide a termination for mooring line 26. Space within pontoon extension 52 not occupied by chain chute 54 may be employed to provide additional buoyancy. This space may comprise one or more watertight compartments.

The AMS mooring lines 26 may be pre-installed and laid out on the seafloor S. The tendons 16 also may be pre-installed and buoyed off. A derrick vessel may be used to deploy the components. The hull 12 may be towed to site and the tendons 16 firstly connected to the hull 12 (or pontoons 14). During hook-up, a derrick vessel may support the hull through its main crane and may effect the connection of the tendons to the hull. After the tendons are connected to the hull and tensioned, the AMS may be attached to the hull, using the derrick vessel to support the main weight of each mooring line (26) and apply tension one line at a time. The AMS of the present invention may be installed and attached to the hull of the TLP either before or after the deck superstructure is installed.

Alternative AMS installation methods are illustrated in FIGS. 5 and 6. As shown in FIG. 5, a winch line 34 from attending vessel V may be attached to the upper end of mooring line 26. Attending vessel V may be dynamically positioned and/or be secured by anchoring means of its own (not shown). A winch on attending vessel V may be used to apply a selected amount of pretension to mooring line 26. Once the desired pretension is applied, a device such as those illustrated in FIG. 7 may be used to secure line 26 with a fixed length.

Another installation method (illustrated in FIG. 6) uses a winch 36 mounted on hull 12 to pretension mooring line 26. This method may employ a single, moveable winch used to successively pretension each mooring line 26 or may employ a plurality of winches which may be used simultaneously or seriatim to apply the desired amount of pretension to each mooring line 26. The methods illustrated in FIG. 5 and FIG. 6 may be used in conjunction. In such a situation, a plurality of winch lines 36 may be attached to mooring line 26 in order to provide the selected amount of pretension. This method may have the added advantage of reducing frictional forces in fairlead 40 on pontoon end 32 as the mooring line 26 passes through the fairlead because the direction of the net force being applied to line 26 may be selectively altered.

Attachment means for mooring line(s) 26 are shown in FIG. 7, a top plan view of the distal end of pontoon 14. FIG. 7 a is a side view thereof, taken along line A in FIG. 7. FIG. 7 b is a partially-sectioned side view of an alternative embodiment taken along line B in FIG. 7. FIG. 7 c is and end view of pontoon 14 taken along line C in FIG. 7. FIG. 7 d is a plan view of hull 12 showing three pontoons or wing 14, each having a pair of mooring line fairleads 40 at its distal end.\

As seen in FIG. 7, pontoon 14 may be equipped with tendon porches 38 for tendon attachment. Additionally, pontoon tip 32 is equipped with pivoting fairleads 40 whose rotation may be restricted as shown in the drawing to prevent entanglement of mooring lines 26. In the embodiment illustrated in FIG. 7 a, chain laydown rack 42 is provided on chain storage area 44 of the upper surface of pontoon 14 to accommodate excess chain comprising at least the end portion of mooring lines 26. An alternative embodiment is shown in FIG. 7 b wherein hawse pipe or chain locker 46 is provided within pontoon 14 for securing excess chain.

As may be seen in FIGS. 7 a, 7 b and 7 c, chain stoppers 48 may be integrated with fairleads 40 to provide a termination of mooring lines 26. Alternatively, chain stopper 48 may be separate from fairlead 40. In certain embodiments, fairlead(s) 40 may be equipped with integral load cell 50 for monitoring the forces on fairlead 40 and/or mooring line 26.

Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 

1. An offshore platform comprising: a buoyant hull; a plurality of tendons each having a first end anchored to the seafloor and a second end attached to the hull such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft; a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end attached to the hull such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.
 2. An offshore platform as recited in claim 1 wherein the hull is a monocolumn hull.
 3. An offshore platform as recited in claim 2 wherein the monocolumn hull has a plurality of radially-extending subsurface pontoons.
 4. An offshore platform as recited in claim 3 wherein the attachment points of the tendons are on each pontoon and the attachment points of the mooring lines are on each pontoon outboard of the attachment points of the tendons.
 5. An offshore platform as recited in claim 4 further comprising a buoyancy compartment on each pontoon between the attachment points of the tendons and the attachment points of the mooring lines.
 6. An offshore platform as recited in claim 5 wherein the buoyancy of the compartment is substantially equal to the vertical component of the force exerted by the mooring lines attached to each pontoon when the tendons of the platform are substantially vertical.
 7. A method of reducing set-down in a tension leg platform comprising: providing a plurality of fixed-length mooring lines each having a first end and a second end; anchoring the first end of each mooring line to the seafloor; and, attaching the second end of each mooring line to the hull such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.
 8. A method as recited in claim 7 wherein the tension leg platform comprises a monocolumn hull.
 9. A method as recited in claim 8 wherein monocolumn hull has a plurality of radially-extending subsurface pontoons.
 10. A method as recited in claim 9 wherein the attachment points of the tendons are on each pontoon and the mooring lines are attached to sites on each pontoon that are outboard of the attachment points of the tendons.
 11. A method as recited in claim 10 further comprising a providing a buoyancy compartment on each pontoon between the attachment points of the tendons and the attachment points of the mooring lines.
 12. A method as recited in claim 11 wherein the buoyancy of the compartment is substantially equal to the vertical component of the force exerted by the mooring lines attached to each pontoon when the tendons of the tension leg platform are substantially vertical.
 13. A method as recited in claim 7 further comprising providing a buoy on each mooring line at a point intermediate the first end and the second end.
 14. A method as recited in claim 13 wherein the buoy is a subsurface buoy or buoys.
 15. A method as recited in claim 7 wherein the mooring lines comprise a section of chain configured to rest on the seafloor when the tendons of the tension leg platform are substantially vertical.
 16. A method as recited in claim 7 wherein the mooring lines are attached to the hull using fairleads which comprise a chain stopper.
 17. A method as recited in claim 7 further comprising pre-tensioning the mooring lines by passing the mooring line through a fairlead attached to the hull and pulling the second end of the mooring line using a winch on an attending vessel.
 18. A method as recited in claim 7 further comprising pre-tensioning the mooring lines by passing each mooring line through a fairlead attached to the hull and hoisting the second end of the mooring line.
 19. A method as recited in claim 18 wherein the hoisting is performed by a derrick barge.
 20. A method as recited in claim 18 additionally comprising applying tension to the mooring lines by also using a winch mounted on an attending vessel.
 21. An offshore platform comprising: a buoyant hull including a generally cylindrical central section and a plurality of subsurface pontoons radially extending from the central section; a plurality of tendons each having a first end anchored to the seafloor and a second end attached to a pontoon such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft; a buoyancy compartment attached to the outboard end of a pontoon such that the buoyancy compartment is substantially outboard of the tendon attachment point on the pontoon; a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end attached to a buoyancy compartment such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.
 22. An offshore platform as recited in claim 21 wherein the buoyancy compartment has a buoyancy that is approximately equal in magnitude to the downward force exerted by the mooring line attached to the buoyancy compartment.
 23. An offshore platform as recited in claim 21 further comprising at least one buoyancy compartment attached to a side surface of a pontoon.
 24. An offshore platform as recited in claim 23 wherein the combined buoyancy of the buoyancy compartment at the end of the pontoon and the buoyancy of the buoyancy compartment attached to a side surface of the pontoon is approximately equal in magnitude to the downward force exerted by the mooring line.
 25. An offshore platform comprising: a buoyant hull including a generally cylindrical central section and a plurality of subsurface pontoons radially extending from the central section; a plurality of tendons each having a first end anchored to the seafloor and a second end attached to a pontoon such that the tendons are loaded in tension and the hull is held by the tendons below its free-floating draft; a buoyancy compartment attached to the outboard end of a pontoon such that the buoyancy compartment is substantially outboard of the tendon attachment point on the pontoon, the buoyancy compartment having an interior passageway extending from an upper surface to a lower surface thereof; a plurality of fixed-length mooring lines each having a first end anchored to the seafloor and a second end extending through the passageway attached to an upper surface of the buoyancy compartment such that lateral movement of the hull is limited to a pre-selected distance and a restoring force is applied by the mooring lines if the hull moves to a position wherein the tendons are not substantially vertical.
 26. An offshore platform as recited in claim 25 wherein the opening of the passageway on an upper surface of the pontoon is generally circular and has a first diameter and the opening of the passageway on a lower surface of the pontoon is generally circular and has a second diameter which is greater than the first diameter.
 27. An offshore platform as recited in claim 25 further comprising a chain stopper on an upper surface of a buoyancy compartment for securing a mooring line comprised of chain at its second end. 